GB2032109A - Measuring temperature and thermal flux - Google Patents

Abstract

For measuring temperatures and temperature differences (and thus thermal fluxes) the temperature- sensitive devices employed are suitable diodes e.g. highly-doped silicon diodes 11, 12 operated at constant current. At constant current, the voltage drop across such a diode varies linearly with temperature, but the rate is strongly current-dependent; thus adjustment of the respective constant current source 15, 16 enables the temperature sensitivity (mV/ DEG C) of the diode to be adjusted to a standard rate. The voltage drop across the diode and part of a potentiometer 13, 14 in series therewith can be standardised at some reference temperature by adjusting the potentiometer tapping. The differential voltage 17 from two diodes, both standardised, indicates their temperature difference, and the thermal flux between them if the conductivity of the material in which they are mounted (e.g. a metal plug filling an aperture in a vessel wall) is known. <IMAGE>

Description

SPECIFICATION Thermal measurement This invention relates to thermal measurement, 'and more particularly to the measurement of temperature and of temperature differences and associated heat fluxes.

It is one object of the invention to provide a sensitive means and a related method of measuring temperature. It is a further object of the invention to provide a sensitive means and a related method of measuring temperature differences and associated thermal fluxes, and particularly smaller temperature differences and smaller associated thermal fluxes than could be measured reliably by means of heat flux meters of conventional thermocouple type.

In the achievement of these objects, according to the invention, use is made of the known temperature dependent characteristics of electrical diodes, by combining such diodes with suitable circuitry to provide the required sensitive measuring means.

The methods and means according to the invention, and the advantages which may be secured by their employment, will be outlined and explained in the following description with reference to the accompanying drawings, in which.

Figure 1 represents a characteristic curve of a semi-conductor diode suitable for use in practising the invention; Figure 2 is a schematic circuit diagram of temperature-difference measuring means according to the invention; Figure 3 is a sectional view through a wall of a thermally lagged vessel containing liquid; Figure 4 is a perspective view, on a larger scale, of temperature-sensing means, according to the invention, provided in the vessel wall shown in Figure 3, and Figure 5 is a schematic circuit diagram of further termperature and temperature-difference measuring means according to the invention.

The invention depends on the use of electrical diodes with suitable temperature-dependent characteristics. Typical of suitable diodes are the highly-doped silicon type, which are readily available as stock items. A diode of this type, carrying a given constant current in its forward direction, exhibits a voltage drop across it which varies almost linearly with diode temperature over a useful temperature range, say from OOC to 1 000C. The rate at which the voltage drop across the diode varies with temperature is typically in the range 2.4 to 3.0 mV/OC, and for a given diode carrying a given current is very dependable and repeatable; but this rate, though constant for a given diode excitation current is markedly dependent on the magnitude of the excitation currrent, in the manner shown in Figure 1.This dependence of the temperature sensitivity on the excitation current affords a ready means of adjusting the thermal performance of a given diode to some predetermined standard, and thus of thermally matching diodes which are to be used in pairs.

In general, adjustment of the thermal sensitivity can be accomplished either by effecting small changes in the excitation current, if the excitation current is itself small (corresponding to the steep part of the curve of Figure 1) or by means of relatively larger changes in the excitation if, corresponding to the flatter part of the curve, a relatively large excitation current is used. The use of small excitation currents tend to lead to problems due to pick-up of external noise unless extensive shielding is employed and capacitive-resistance filter networks are provided in the constantcurrent supply, and it is therefore preferred to use relatively large excitation current even though that requires the provision of a constant-current supply source which is capable of a greater range of adjustment of its constant-current output.The diode excitation current must in any event be limited to values which will not result in appreciable self-heating of the diode (a maximum excitation-current value of about 90"A being preferred for that reason) and constant-current supply units with control of the output over a range up to that maximum are, in fact, commercially available.

Figure 2 is a schematic circuit diagram of measuring equipment in accordance with the invention for measuring temperature differentials, and of an auxiliary unit used in normalising the thermal performances of two diodes of the measuring equipment, and thus matching the diodes to one another. As shown, the equipment comprises a first diode 11 and a second diode 12 each connected in series with a respective potentiometer resistor 13 or 14 to be supplied therethrough with a forward excitation current by a respective variable-output constant-current source 15 or 16.The negative sides of the two diodes and their excitatiton current sources are strapped together, and variable tapping points X and Y of the potentiometers 1 3 and 14, respectively, have a sensitive voltmeter 1 7 connected between them in use of the equipment.

Before use of the equipment, however, the characteristics of the diodes 11 and 1 2 must be matched to one another; and that, preferably, is done by normalising the characteristics of each diode separately by reference to an auxiliary unit 1 8 which comprises a constant-voltage supply source 1 9 and a potentiometer 20 across which the output of the source 1 9 is applied.To normalise the characteristic of the diode 11, that terminal of the voltmeter 17 which in Figure 1 is shown connected to the tapping Y of the potentiometer 14 is temporarily connected instead to a -variable tapping Z of the potentiometer 20, of which the negative end is temporarily connected (as indicated by a broken line) to the negative sides of the diodes 11 and 12.

With the tapping X initially set near the midpoint of its adjustment range and the tapping Z set to provide some standard fixed reference voltage, the diode 11 is brought to two reference temperatures (say 300 and 400 C) alternatively while the constant-current output of the source 1 5 is adjusted until the swing in the reading of the meter 1 7 due to the controlled swing in the temperature of the diode is set at a desired amount. Thus 2.4 mV/OC may be chosen as a suitable standard thermal slope for the diodes, and the diode 11 may be standardised by so adjusting the output of the source 1 5 that the ten-degree temperature swing of the diode between 300 and 400C produces a change of 24mV in the reading of the meter 17.Then, with the source 15 thus adjusted, the tapping X may be adjusted to bring the meter reading to some standard value when the diode 11 is at some standard temperature (such as 300C). The thermal slope of the diode 12 may then be similarly standardised while the meter 1 7 is connected between the tappings Y and Z and the output of the source 1 6 is adjusted as necessary (to a value which will in general be different from the adjusted output value of the source 15, since the diodes 11 and 12 will in general be different from one another); and the tapping Y may then be adjusted to bring the meter reading to the same standard value, at the same standard temperature, for the diode 1 2 as for the diode 11.Thereafter, with the meter 1 7 connected between the tappings X and Y as shown in Figure 1, the meter reading will be representative, on a scale determined by the selected thermal slope of the diodes 11 and 12, of the temperature difference between the two diodes.

The use of a pair of diodes in this way for the sensitive measurement of temperature difference may be utilised, for example as now to be described with reference to Figures 3 and 4, in measuring thermal flux through a vessel wall and in testing the effectiveness of thermal insulation provided for the vessel.

Figure 3 shows part of a wall 21 of a vessel which contains hot liquid 22 from which heat is lost through the vessel wail at a rate dependent on the effectiveness of a layer 23 of thermally insulating material surrounding the vessel. The insulating layer may be applied directly to the wall 21, or there may be interposed (as shown) a space which is filled with liquid 24 which tends to maintain uniformity of temperature over the outer surface of the wall 21. The wall 21 is formed with circular apertures 25, each of which is closed by a respective cylindrical plug 26 of the same material as the vessel (e.g. cut from stainless steel bar if the vessel wall is of stainless steel). Each plug 26, as shown more clearly in Figure 4, is formed with four parallel bores 27, 28, 29 and 30.The bores 27 and 28 are near to, and parallel to, one end face 31 of the plug, and equally spaced on opposite sides of a diameter parallel to both; and the bores 29 and 30 are similarly disposed relative to the other end face, 32. Each end of each bore opens into an end of a respective channel 33 cut in the cylindrical surface of the plug 26 and extending to the end face 32 thereof.Centrally disposed in each bore is a respective diode of the kind referred to above; thus, for example, the bores 27 and 29 are shown in Figure 4 as accommodating the diodes 11 and 1 2 respectively of Figure 2, while the bores 28 and 30 accommodate a pair of stand-by or substitute diodes 1 and 12' which may be connected in circuit in place of the diodes 11 and 12 if either of these latter should cease to function satisfactorily due to aging or other cause. The two leads of each diode, suitably insulated from the material of the plug 26 if that is electrically conducting, are brought out to opposite ends of the respective bore in which the diode is housed and thence, accommodated in the respective channels 33, out beyond the end face 32 of the plug.To receive diodes of 1.5 mm diameter, the bores 27 to 30 may be of some 1.78 mm diameter: and the channels 33 may be of smaller cross-section since they only have to accommodate the leads. The remaining space in the bores and channels is preferably filled with, for example, a one-part epoxy resin which combines very high electrical insulation resistance with appreciable thermal conductivity. This latter may be only some 5% of that of stainless steel, but the low thermal conductivity only increases the response time, as heat fluxes are progressively reduced, and does not affect the steady-state accuracy of measurement.

The plug 26 may suitably be of 20 mm diameter and secured in the aperture 25 by using an interference fit, of about 0.04 mm. In large scale production of such plugs, it might be convenient to form the diode leads in the channels 33 by flame spraying, thus enabling the channels 33 to be of minimal cross-section.

In a practical application of the invention, a single pair of constant-current sources 1 5 and 1 6 might be connectable to any selected pair of diodes from a substantial number of such pairs; for example, a test vessel like the vessel 21 of Figure 3 might be provided with several plugs 26 distributed over its wall area and each containing a pair (or, as described above, two pairs) of diodes.

Then, suitably, a circuit arrangement like that represented schematically in Figure 5 may be employed. As represented in Figure 5, any one of a plurality of diodes 11 a to 1 1f (each in series with a respective one of a plurality of potentiometers 1 3a to 1 3f having adjustable tappings Xa to Xf respectively) may be selected by selector means 35 to receive the constant-current output of the source 15; and similarly any one of a plurality of diodes 1 2a to 1 2f (each in series with a respective one of a plurality of potentiometers 1 4a to 1 4f having adjustable tappings Ya to Yf respectively) may by selected by selector means 36 to receive the constant-current output of the source 1 6.

Selector means 37 enables one input terminal of the voltmeter 17 to be connected to any one of the potentiometer tappings Xa to Xf, and selector means 38 similarly enables the other input terminal of the voltmeter 1 7 to be connected to any one of the tappings Ya to Yf. The selector means 35, 36, 37 and 38 are controlled by a microprocessor unit 39 programmed to provide that when the selector means 35 selects any particular one of the diodes 1 1a to 1 If, say 1 a, the selector means 36 selects that one of the diodes 1 2a to 1 2f with which it is paired (thus 1 2a with 11 a) and the selector means 37 and 38 select the corresponding ones of the tappings Xa to Xf and Ya to Yf.Each of these tappings is preadjusted to give the required voltage off-set (as described above with reference to Figure 1); and the microprocessor, which is provided with'a readonly memory storing the value of the constantcurrent drive required by each of the diodes, is arranged to control the sources 1 5 and 16 so that the output of each is correct for the respective pair of diodes selected by the selector means 35 and 36. As described above, the circuit of Figure 5 provides for differential temperature measurements between the selected pair of diodes, and thus heat flux measurements since the separation of the diodes and the intervening material and its thermal conductivity are known. It will be apparent that provision may be made for operatingthe microprocessor in another mode (viz.

to cause the selector means 38 to select a nosignal connection 40) so as to cause the meter 1 7 to register actual temperature (as detected by a selected one of the diodes 11 a to 11 f) instead of temperature differential.

Claims (7)

1. A temperature sensing device comprising a solid-state electrical diode, a relatively temperature-insensitive potentiometer, a variableoutput constant-current generator having the diode and the potentiometer connected or connectable in series to conduct its constantcurrent output current in the forward direction of the diode, and a voltage-sensing device connected or connectable between a variable tapping of the potentiometer and a point at a reference voltage.

2. A temperature-sensing device as claimed in Claim 1, wherein the diode is of the highly-doped silicon type.

3. A method of meaturing temperature comprising adjusting the temperature sensitivity of a temperature-sensing device as claimed in Claim 1 or Claim 2 by adjusting the output current of the constant-current generator thereof, adjusting the voltage applied to the voltage-sensing device at some chosen temperature by varying the potentiometer tapping while maintaining the diode at that temperature, and thereafter obtaining an indication of the voltage applied to the voltage-sensing device as an indication of the temperature for the time being of the diode.

4. A temperature-differential sensing device comprising a temperature-sensing device as claimed in Claim 1 or Claim 2 and further comprising a second solid-state electrical diode, a second relatively temperature-insensitive potentiometer, and a second variable-output constant-current generator having the second diode and the second potentiometer connected or connectable in series to conduct its constantcurrent output in the forward direction of the second diode, the said point at a reference voltage, to which the voltage-sensing device is connected or connectable, being constituted by a variable tapping of the second potentiometer.

5. A method of measuring temperature differentials comprising adjusting the temperature sensitivity of each temperature-sensitive diode of a device as claimed in Claim 4 to a chosen value by adjusting the output current of the respective constant-current generator, separately adjusting to a chosen value the voltage output to the voltage-sensing device from each such diode at some chosen temperature thereof by varying the respective potentiometer tapping while maintaining the respective diode at that temperature, and thereafter, with the voltagesensitive device connected between the adjusted potentiometer tappings, obtaining an indication of the voltage applied across the voltage-sensing device as an indication of the difference between the temperatures for the time being of the respective diodes.

6. A temperature-sensing or temperaturedifferential sensing device as claimed in Claim 1 or Claim 4 and substantially as described herein with reference to Figures 1 2,3 and 4 or 5.

7. A method of measuring temperature or temperature differentials employing a sensing device as claimed in Claim 6 and substantially as described herein.